Low power lasers are employed to read and write binary data on a data region of media. One typical media are optical storage discs, such as CDs, DVDs and the like. Typically, various types of data are written on a data side of the disc by a laser beam while the disc is rotating. Data may be recorded by changing a property of a desired area on the recording media so that the area is indicative of a zero or one data value.
The side of a data disc opposite the data side is often used as a label region for handwriting or affixing or marking a label with descriptions and illustrations corresponding to the recorded data. Recently, apparatus and methods have been developed with the ability to generate an optically visible label on the label region of an optical disc using the same laser that was employed to read and write digital or electronic data on the data side of the disc. See U.S. Patent Application Publication No. 2003/0108708 (Anderson, et al.), disclosing the use of laser sensitive materials on a disc label that react chemically with the application of light and heat and result in changes in color and shading on the label. As used herein, the terms “spot” or “data spot” refers to a non-visible spot or mark made on the data region of the media. The terms “mark” or “visible mark” refers to visible mark or spot made on the label region of the media.
In making data spots using an optical disc drive, small spots are produced extremely closely together, in order to pack as much data as possible onto the disc. Typically data spots are only about 1.7 micron in diameter or less. Thus, it is important to provide precisely focused data beams that are positioned exactly on the appointed data track. Another reason for having a focused beam for data writing is that a protective surface layer and/or defects and dust are commonly part of the data surface of a medium. Hence, a focused beam is needed for data writing to allow the beam to focus on the active layer. Likewise, in reading data spots, the read beam must be precisely focused and tracked to obtain correct data. This data read/write process requires precise and expensive optics and tracking elements in the data marking and optical pickup unit (OPU) devices of a laser/optical disc system, adding substantially to the cost of such systems.
In addition, it is important that data spots be small enough to enable suitable data storage density. Accordingly, it is desirable that a spot on the optical disc surface be made as small as possible to ensure accurate writing and reading. Numerical Aperture (NA) and wavelength of the electromagnetic radiation determine the minimum data spot size that can be marked on an optical data storage media. The smallest data spot size is created by using the highest practicable NA for the objective lens, that is the lens that focuses the laser light onto the media. For data systems, the entrance NA of an optical data system next to the data writing laser, referred to as the entrance pupil, has typically been small (less than 0.10). This small NA is chosen to make the irradiance distribution on the back of the objective lens as even as possible. The exit NA of the optical data system next to the recording surface, called the exit pupil, has typically been relatively large (more than 0.45), so that the data spot can be very small when focused. The small NA of the entrance pupil results in a substantial loss of laser power, as most of the light does not pass through the optical system. Existing OPUs are about 37% efficient from the laser diode output to the data spot. In addition, the large exit NA of an optical data system provides a short range of working distance within which the spot is in focus, thereby causing narrow tolerances in the variation of allowable distances between the optics and recording media.